This invention relates generally to an electrical instrument and, more particularly, to a control mechanism for adjusting a variable electrical characteristic of an electrical instrument.
Many electrical instruments such as electrical temperature controllers must be calibrated to insure accurate performance. Typically, such calibration entails adjusting the control shaft of a component that provides a variable electrical characteristic such as resistance, capacitance or inductance. Normally, the control shaft adjustment is made with respect to an appropriate indicator scale and while monitoring a known standard. Most control instruments employ a control knob that functions both as a means for manually adjusting a control shaft and as an indicator dial for an associated scale. Calibration is generally achieved by freely rotating the knob on the control shaft until appropriate registration between the knob and scale is attained and then locking the knob in that position on the shaft with a conventional set screw. A significant drawback of this technique stems from the periodic requirement for removing the control knob to permit a thorough cleaning of the instrument's control panel. Each such removal disturbs the previously established calibrated registration between the control knob and indicator scale thereby necessitating a recalibration of the instrument.
The object of this invention, therefore, is to provide an electrical instrument having a control assembly with a control knob that can be rotationally aligned during a calibration process and subsequently removed without disturbing that calibration.